Method for dissolution of chromium from chromite ores



Patented June 24, 1952 METHOD FOR DISSOLUTION OF CHROIHIUM FROM CHROMITEORES Rex R. Lloyd, Boulder City, Nev., and William T. Rawles,Pittsfield, Mass, assignors to the United States of America asrepresented by theSccretary of the Interior No Drawing.

2 Claims.

This invention relates to a method for the dissolution of chromiumbearing ores, and it particularly relates to the dissolution of ores ofthe chromite series, chromium spinel ores, and similar .difiicultlysoluble chrome ores. Still more particularly the invention relates to amethod for the dissolution of such chromium ores using the spent anolytefrom a process for the electrowinning of chromium.

Chromium ores such as those of the chromite series (Dana's The System ofMineralogy, 7th edition, vol. I, page 709), spinel series (page 689 ofthe aforementioned reference) and like ores can be dissolved only withgreat difficulty. It has been found possible to digest such ores inconcentrated sulfuric acid (1. e., greater than 65% by weight H2804),and it is also necessary to employ strong oxidizing agents such ashexavalent chromium compounds, lead peroxide, manganese dioxide or thelike in order to effect complete dissolution of the chromium valuesthereof. Whenever a solution containing ammonium ions is employed fordissolving the chrome ore, as in the case where the spent anolyte fromprocesses for'the electrowinning of chromium is so used, a highlyinsoluble iron ammonium complex, probably a ferric ammonium complexforms during the digestion. This complex appears to contain or occlude aconsiderable portion of chromium and also causes chromium losses due toits tendency to form on and coat the ore particles, thereby preventingdissolution.

Accordingly, it is an objectof this invention to provide a simple andeconomical method for the dissolutionof certain diflicultly solublechromium ores. 1

It is another object of the invention to provide a method for thedissolution of chrome ores when the solvent contains ammonium ions,without concurrently forming insoluble metallic ammonium complexes.

Other objects and advantages will be apparent or will appearhereinafter.

These objects and advantages are accomplished in accordance with thisinvention wherein the chromium ore is digested in a solvent comprising adilute sulfuric acid solution containing an oxidizing agent atsuperatmospheric pressures.

Suitable chrome ores for treatment in accordance with this inventioninclude those of the aforementioned chromite series such asmagnesochromite, chromite, and the like; those of the spinel series suchas chromium spinel, chro- Application February 24,1948, Serial No.10,552.

(Granted under the act of March '3, 1883, as amended April 30, 1928;3'70 0. G. 757) 2 mian hercynite and the like; and other chromium oreswhich exhibit some degree of solubility in sulfuric acid, in thepresence of an oxidizing agent.

,In operation the chromium ore is preferably pulverized so that it willhave a greater surface upon which the solvent can act. This apparentlyis not a critical factor to the operation of the invention, andsatisfactory results have beenobtained bygrinding the ore or a chromiumore concentrate to sizes less than about mesh. The speed of thereaction, howeverjis a function of particle size. The groundore ischarged into a chemically resistant reaction vessel capable ofwithstanding the pressures and temperatures involved. A stoichiometricequivalent of dilute sulfuric acid (1. e., acid not greater than about50 per cent by weight and preferably stronger than about 20 per cent byweight) and containing an amount of chromic acid equal to about 2 to 7per cent of the Weight of the sulfuric acid; or an equivalent amount ofother oxidizing agents such as lead peroxide, manganese dioxide, and thelike is mixed with the charge. The vessel is then sealed and thepressure is permitted to build up to between about 30 and 90 p. s. i.gage. This pressure will vary inversely with the acid concentration ofthe solvent. Since the reaction is exothermic, it will proceed withoutthe application of external heat and pressure will depreferably betweenabout to degrees centigrade. The completion of the digestion will beindicated by a drop in temperature of the reaction mass. The solutionproduced in this mannerwill be found to contain substantially all of thechromium in a soluble form. If ammonium ions are present in thesolution, as might be as hereinafter described, no insoluble metallicammonium complexes will be formed under the conditions outlined above.

In a preferred embodiment of the invention the spent anolyte from cellsfor the electrowinning of chromium is employed as the solvent for thechromite. The anolyte as discharged from such cells, generally containsabout 300 grams per liter of sulfuric acid, 15 grams per liter ofammonia as ammonium sulfate, and 20grams of chromium as chromic acid.The acid-concentration of-the spent anolyte'is built up to about45 percent sulfuric acid, using raw'sulfuric acid and,

3 if necessary, suiiicient chromic acid is added to bring the totalchromic acid to about 2 to 7 per cent of the weight of the totalsulfuric acid concentration. Batches of low grade chromite concentratesare wet ground to about minus 270 mesh and dried in any suitable manner.The ore is digested in batches using a stoichiometric quantity of thefortified anolyte. Such digestion has been carried out conveniently inan acid-resistant brick-lined digestor capable of withstanding pressuresas great as 100 p. s. i. The temperature during digestion is maintainedat between about 130 to 150 degrees centigrade, and

this control was achieved by maintaining the the raw digest liquor isremoved as a normal metal sulfate solution and containing practical- 1yno excess sulfate radical.

The raw digest liquor contains various ionic constituents in addition tothe chromium and sulfate; e. g., iron, aluminum, etc. The term normalmetal sulfate solution is used to indicate that the amount of sulfatepresent is that stoichiometrically necessary to form normal sulfates ofthe various metals present in the raw digest liquor.

When it'is desired to recirculate this regenerated solution through theelectrolytic cells, it is necessary to purify the raw digest liquor.This can be accomplished by diluting the raw digest liquor with spentmother liquor obtained in a subsequent crystallization process describedhereinafter to produce a solution containing about 42 grams of chromium(calculated as elemental chromium) and 150 grams of ammonium sulfate perliter, appreciable quantities of iron, aluminum, magnesium, and sodium,and small quantities of several other metals. This diluted solution isfiltered at 65 degrees centigrade to remove siliceous gangue and heatedto so degrees centigrade for one hour. The liquor is sent immediately toa vacuum crystallizer, where all of the aluminum and most of the ironand magnesium are removed at 4 degrees centigrade.

These ammonium sulfate complex crystals contain most of the leachedimpurities. Chromium remains in solution because of the above-mentionedprolonged heat treatment. These impurity crystals are removed inrubber-lined basket centrifuges, and the purified mother liquor isclarified in a wooden filter press and sent to aging.

Aging takes place in a Koroseal-lined thickener at 30 degreescentigrade. Chromium reverts from the green to the violet modificationand crystallizes as chromium ammonium alum, containing some ferric ironand small quantities of many other impurities. The chrome alum isremoved by centrifuging; and the spent mother liquor containing about 22grams chromium (calculated as elemental chromium) per liter, can be usedto dilute the raw digest liquor as aforementioned.

One more crystallization of the chrome alum in a thickener at 30 degreescentigrade, using spent catholyte from the electrowinning cells asrecrystallizing. liquor, reduces iron and other impurities below thecritical concentrations. The iron in the crude alum is reduced to theferrous state by the divalent chromium in the spent catholyte and doesnot recrystallize with the chromium. The pure chrome alum is mixed withglaubers salts, recovered from the impurity alums, and dissolved toproduce cell feed containing grams of chromium (calculated as elementalchromium), grams of ammonium sulfate, and 60 grams of sodium sulfate perliter.

The following examples will show how the invention may be carried out,but the invention is not limited thereto.

Example I A sample of 500 grams of chromite was charged into a jacketedstainless steel autoclave and a solution was added which contained 1,250grams of sulfuric acid, 2,325 grams of water, 45 grams of hexavalentchromium, and 34 grams of ammonia as ammonium sulfate. The chromite wasdigested in this 35 per cent acid for 6 /2 hours at 80 pounds pressureand 165-175 degrees centigrade. At the end of this period, 63 per centof the chromium in the ore was rendered water soluble and no dehydratedalum was produced.

When this process was repeated, using a 65 per cent sulfuric acidsolution, a large quantity of insoluble alum was produced whichcontained substantially all of the chromium.

Example II A brick-lined digestor was constructed with a pressure coverbut without any means of applying external heat. This digestor wascharged with 480 pounds of chromite ground to 98 per cent minus 325 meshalong with a solution containing 39.5 pounds of hexavalent chromium,12.5 pounds of ammonia as ammonium sulfate and 210 pounds of sulfuricacid. The digestor was closed and an additional 960 pounds of acid waspumped in, making a 45 per cent sulfuric acid solution. The temperaturebegan to rise and at the end of 30 minutes was 145 degrees centigrade,and the pressure was 38 pounds gage. Steam, and some inert gas, was bledoff gradually to keep the temperature at to degrees centigrade. After 10hours, the temperature dropped to 100 degrees centigrade, the digestorwas opened and the solution tested. Analysis showed that 23 pounds ofacid remained and 99 per cent of the chromium in the ore was watersoluble. No dehydrated alums were present.

From the foregoing description and examples it is apparent that a simpleprocess for the dissolution of chromite in the spent anolyte fromelectrolytic cells for the electrowinning of chromium has beendeveloped.

While the invention has been particularly described for the digestion ofchromite ores, it is not limited thereto; other chromium ores exhibitingsolubility characteristics in sulfuric acid can also be dissolved bythis process.

While the invention has been particularly described as employinghexavalent chromium as the oxidizing agent, it is not limited theretoand other oxidizing agents can be substituted therefor.

Since many widely differing embodiments of the invention will occur toone skilled in the art, the invention is not limited to the specificdetails illustrated and described, and various changes can be madetherein without departing from the spirit and scope thereof.

What is claimed is:

1. A method for the dissolution of chrome ores in a solution containingammonium ions without the formation of chrome ammonium alum whichcomprises dissolving said ores in a solution containing ammoniumsulfate, chromic acid, and to per cent sulfuric acid by weight, saidsolution having a temperature within the range of 165 C. to C'., thetemperature decreasing with the increase in percentage of sulfuric acid.

2. In a process for the electrowinning of chromium from an electrolytewhich contains trivalent chromium in the presence of sulfate ion andammonium ion, the improvement which comprises regenerating the chromiumconcentration of the spent electrolyte which contains chromic ion,ammonium ion and sulfate ion by increasing the sulfuric acidconcentration to from 30 to 50 per cent by weight, and dissolving chromeore in the thus formed solution at a temperature within the range of C.to 125 C., the temperature decreasing with the increase a in percentageof sulfuric acid.

REX R. LLOYD. WILLIAM T. RAWLES.

6 REFERENCES CITED- The following references are of record in the fileof this patent:

UNITED STATES PATENTS Number Name Date 743,668 Sucky et a1. Nov. 10,1903 1,403,960 Hultman Jan. 17, 1922 1,502,035 Hasenclever July 22, 19241,723,536 Weise Aug. 6, 1929 1,749,443 Proctor Mar. 4, 1930 2,507,476Lloyd May 9, 1950 FOREIGN PATENTS Number Country Date 187,636 GreatBritain Nov. 2, 1922 OTHER REFERENCES Lloyd, Transactions of theElectrochemical Society, vol. 89, pages 443-454, 1946.

1. A METHOD FOR THE DISSOLUTION OF CHROME ORES IN A SOLUTION CONTAININGAMMONIUM IONS WITHOUT THE FORMATION OF CHROME AMMONIUM ALUM WHICHCOMPRISES DISSOLVING SAID ORES IN A SOLUTION CONTAINING AMMONIUMSULFATE, CHROMIC ACID, AND 30 TO 50 PER CENT SULFURIC ACID BY WEIGHT,SAID SOLUTION HAVING A TEMPERATURE WITHIN THE RANGE OF 165* C., THETEMPERATURE DECREASING WITH THE INCREASE IN PERCENTAGE OF SULFURIC ACID.